I just found this thread searching to see if anyone had gotten this to work. I'm on Linux with an nvidia card and 3 displays which requires the use of xinerama (actually, I have two triple head linux setups one with a NV GTX 770 card driving 3 displays another other two NV NVS 290 cards with one card driving 2 of the displays -- it doesn't work on either setup)
I suspect that the issue is spring's dual monitor support only works in full screen mode and due to the way X R&R + xinerama works "full screen" is inherently limited to a single X display. I suppose I should try disabling xinerama and testing just NV twinview across two displays.
What I'd ideally like is to be able to pull the minimap off into it's own window so I could still use one display for lobby + irc + etc, one for an enlarged minimap, and the 3rd for a maximized spring window.
As a side note, stretching a spring window across multiple 1920x1200px displays really kills performance. I suspect something is doing software rendering (even with LUPS/etc. disabled) that is sensitive to raw resolution.